Electronic measurement devices typically comprise one or more sensors which are arranged to sense at least one physical phenomenon. This physical phenomenon may be a voltage, a current, a temperature, a humidity-level, a CO2-level, a pH-value, and atmospheric pressure, for example. Besides the sensors electronic measurement devices typically comprise signal conditioning, conversion and processing circuitry. In addition, the measurement devices comprise interfaces for conveying the results of measurements of physical phenomena to a user. These interfaces may consist of a display or a wired or wireless communication link. The user could, for example, be a person or an autonomous control or monitoring system controlling or monitoring the physical phenomena in a certain environment, such as a smart building.
Most measurement devices require some form of calibration. Calibration may be defined as follows: “Calibration is a comparison between measurements—one of known magnitude or correctness made or set with one device and another measurement made in as similar a way as possible with a second device. The device with the known or assigned correctness is called the standard. The second device is the unit under test, test instrument, or any of several other names for the device being calibrated.”(http://en.wikipedia.org/wiki/Calibration).
In the context of the present application the first device is called the “calibration device” and the second device is called the “measurement device”. It is noted that a calibration device may in fact be regarded as a specific type of measurement device which has an already known or assigned correctness, whereas the extent to which a general measurement device is correct or incorrect must still be determined. A determined degree of incorrectness—or deviation—is used to compute so-called calibration parameters. Subsequently, during normal operation, a measurement device uses the calibration parameters to correct sensor values into more accurate measurements. Correcting these sensor values is typically done by processing circuitry within the measurement device.
Calibration may be needed during the manufacturing of the measurement device as well as during its normal operation, for example periodically. The former need may be caused by tolerances in the fabrication process of sensor, signal conditioning circuitry and/or signal conversion circuitry and the latter may be caused by lifetime aging of these components.
A wireless sensor node is an example of an electronic measurement device that may require calibration. As its name indicates, a wireless communication link is used as means to convey measurement results to a user. The user may be a person, but it may also be an automated control or monitoring system. In many applications wireless sensor nodes may be combined into large wireless sensor networks (WSNs). For example, it is expected that in the future modern office buildings will comprise up to one sensor per square meter to ensure optimal comfort for its inhabitants at the expense of minimal energy. This means that a WSN in such buildings may span thousands of nodes, i.e. measurement devices. Similar developments are anticipated in other applications such as infrastructural monitoring, health care, and agriculture.
Unfortunately, the calibration of a measurement device may be time-consuming and error prone. When thousands of measurement devices—such as wireless sensor nodes in buildings—must be calibrated periodically, the costs of calibration may be substantial. Therefore, there is a need to reduce the calibration effort to a single and relatively simple end-user action.
Furthermore, if different calibration devices and/or procedures are required for different types of measurement devices, the task of calibrating a large number of such measurement devices is quite complicated. For example, different wireless sensor nodes addressing different physical phenomena in a building all need to be calibrated. There is a need to reduce the calibration efforts also in these cases.